This invention relates to improved, low frequency bandpass loudspeaker systems.
In the art of loudspeaker enclosures there are two basic types of systems that are most common. The sealed or acoustic suspension system, which consists of an electroacoustical transducer mounted in an enclosed volume that has the characterization of acoustic compliance. The second type is commonly called a bass-reflex system which includes an electroacoustic transducer mounted in an enclosure that utilizes a passive acoustic radiator or vent having the characteristic of acoustic mass which interacts with the characteristic acoustic compliance of the enclosure volume to form a Helmholtz resonance. A reflex system (enclosure/vent—compliance/mass) that exhibits a Helmholtz resonance shall be referred to hereinafter as a Helmholtz-reflex.
One of the prior art configurations relevant to the invention is the multi-chamber bandpass woofer system. Historically it has been shown that for a given restricted band of frequencies an acoustical bandpass enclosure system can produce greater performance both in terms of the efficiency/bass extension/enclosure size factor and large signal output compared to non-bandpass systems such as the basic sealed or bass reflex enclosures. The basic forms of these bandpass systems are discussed in the literature. See for example ‘A bandpass loudspeaker enclosure’ by L. R. Fincham, Audio Engineering Society convention preprint #1512, May.
The earliest patent reference to a “single” Helmholtz-reflex tuned bandpass woofer system is Lang, ‘Sound Reproducing System’ U.S. Pat. No. 2,689,016. This patent reference embodies the most common version of bandpass woofer system that is used in many systems today. This type of system includes an enclosure with two separate chambers with an active transducer mounted in a dividing panel separating and communicating to both chambers. One chamber is sealed, acting as an acoustic suspension and the other is ported, operating as a vented system with a passive acoustic mass communicating to the environment outside the enclosure.
The single tuned prior art bandpass woofer systems suffer from a number of shortcomings. First, they tend to have a series of resonant amplitude peaks that appear above the pass band of the bandpass system. These are due to standing waves in the enclosure chamber and are well documented in the article by Fincham listed above. Prior art solutions to this problem suggest the use of damping materials which unfortunately damp out useful system output at the same time they damp out the undesired resonances. Secondly, they have a cone excursion minimum at their Helmholtz-reflex frequency but there is only one tuning and it is placed at a frequency near the highest frequency of interest where cone excursion is insignificant compared to the lower frequency range of the system. If the vent tuning is placed at a lower, more useful frequency then the system suffers from reduced high frequency bandwidth.
The next evolutionary step in complexity of a prior art bandpass woofer is expressed in the earliest patent reference to a “dual” Helmholtz-reflex bandpass woofer system in FIG. 1 in D'Alton, ‘Acoustic Device’ U.S. Pat. No. 1,969,704. This reference discloses an enclosure containing a two chamber bandpass woofer system with an active transducer mounted in the dividing panel and communicating to both chambers. Each chamber has a passive acoustic radiator communicating to the environment outside the enclosure. European patent 0125625 ‘Loudspeaker enclosure with integrated acoustic bandpass filter’ by Bernhard Puls and U.S. Pat. No. 4,549,631 ‘Multiple porting loudspeaker systems’ granted to Amar G. Bose are derived from the same basic structure as shown in the D'Alton reference.
An alternative arrangement of a dual Helmholtz-reflex bandpass system is disclosed in the U.S. Pat. No. 4,875,546 ‘Loudspeaker with acoustic band-pass filter’ granted to Palo Krnan. This system includes an enclosure with two separate chambers with an active transducer mounted in the dividing panel there between and communicating to both chambers. One chamber is ported with a passive acoustic radiator communicating to the environment outside the enclosure. There is a second passive acoustic radiator communicating internally between the two chambers.
These dual tuned bandpass subwoofers suffer from the same out of band, high frequency chamber resonances that are endemic to the single tuned bandpass system. Further, by venting the lowest frequency chamber and tuning it to a lower frequency, the vent length tends to be longer and therefore produce vent/pipe resonances which can be quite audible as a distortion of the original signal.
U.S. Pat. No. 5,092,424 ‘Electroacoustical transducing with at least three cascaded subchambers’ granted to Schreiber et al. is an extension of the above listed bandpass art. It utilizes an enclosure with at least three chambers such that it is substantially equivalent to the Bose '631 patent listed above, but with an additional enclosure volume added to the outside of the main enclosure. This additional enclosure receives the two ports from the internal main chambers and an additional passive acoustic radiator communicates to the environment outside the system. This system suffers from the same low frequency vent resonance problems as the dual tuned bandpass systems.
Each of the above patents have shortcomings that have limited the full potential of the bandpass approach for low frequency reproduction. In general, the above systems suffer from either a slow lowpass cutoff in the higher frequencies, where the greatest extension with the sharpest cutoff is most desirable, or unattenuated, higher frequency resonances which can cause audible distortion.
In a co-pending patent, the inventor eliminated vents from the low frequency chamber in multi chamber bandpass systems partially to avoid the pipe resonances that are generated from prior art bandpass systems with vented low frequency chambers. The inventor has found the shortcomings of prior art systems can be overcome by the novel vent/enclosure arrangement disclosed herein.
It would be desirable to have a woofer system that combined an extended frequency, steep slope lowpass characteristic at the high frequencies while at the same time having a Helmholtz-reflex tuning at the lowest frequency filtering out any resonance or distortion resulting from the lowest frequency passive acoustic radiator.